1. Field of the Invention
This invention relates to acoustic signal transmission through solids, especially through metal pipe such as a drill string. More particularly, it relates to an electrohydraulic transmitter releasably attachable to a drill string at the surface to send selected, coded frequencies of longitudinal, acoustic waves downward through the drill string to a subsurface receiver.
2. The Prior Art
Telemetry is a major research area for rotary drilling operations. A reliable system to communicate to or from a subsurface equipment package is coveted by most individuals involved in the art. Numerous solutions for transmitting information have been studied. One approach employs a physical communication line to transmit signals by mechanical, hydraulic, pneumatic, or electrical pulses. Use of these systems is costly in terms of the outlay for materials and of the installation and operational costs.
Others have tried to avoid the problems of communication lines within the drill string. In U.S. Pat. No. 3,737,845 (Maroney et al.), electrical signals are transmitted from the surface to a subsurface receiver through the strata surrounding the wellbore. In U.S. Pat. No. 4,078,620 (Westlake et al.), binary coded pressure pulses are transmitted from a subsurface equipment package to the surface in the drilling mud. The pulses are created by venting drilling mud through a valve in the drill string stem. Acoustic telemetry has also been studied. At least three approaches are used. Acoustic pressure waves may travel between surface and subsurface locations in the strata surrounding the wellbore. See, e.g., U.S. Pat. No. 3,876,016. The acoustic waves may travel through the drilling mud. Signals in the mud are usually produced by a downhole turbine generator. See, e.g., U.S. Pat. Nos. 3,233,674; 4,100,528; or 4,103,281. The acoustic waves may travel through the drill string tubulars. Because this invention relates to signal transmission in the drill string, this third method will be discussed in greater detail.
In U.S. Pat. No. 3,103,643 Kalbfell discloses a downhole, electromechanical transmitter which produces acoustic waves of low frequency by vibrating adjoining pipe sections. Similarly, in U.S. Pat. No. 3,252,225 Hixson discloses a compression-wave mechanical generating system which produces low frequency, longitudinal acoustic waves by the contact of an oscillating mass with the inside of the drill string. Hixson's apparatus uses a spring and weight principle to control the frequency of the longitudinal (compressional) waves generated. The spring and weight store a burst of energy which is released whenever drilling mud circulation stops.
A somewhat different concept is revealed in U.S. Pat. No. 3,889,228. Shawhan discloses use of a series of acoustic repeaters, preferably piezoelectric accelerometers, to signal to and from a downhole equipment sub with acoustic waves of approximately 1000 Hertz (Hz). Because the signals attenuate over the relatively large distances that the acoustic waves must travel, amplification is required. Repeaters allow for transmission over greater lengths. Nevertheless, they substantially increase equipment costs; they increase handling costs; and they reduce reliability of the system.
Repeaters may be eliminated if suitable frequencies are used. Hixson discusses the advantages of low frequencies, particularly those at which the typical pipe length equals an odd number of one-quarter wavelengths. Barnes and Kirkwood use a more sophisticated model of a drill string in theorizing its two lowest passbands between 0 and 280 Herts (Hz) and between 330 and 570 Hz for optimal transmission of longitudinal acoustic waves. Barnes and Kirkwood, Passbands for Acoustic Transmission in an Idealized Drill String, 51 J. Acoustical Soc'y Am. 1606 (1972). To produce lower frequencies requires powerful transmitters. Power requirements limit the feasible frequencies attainable by downhole devices. Repeates may be necessary with any downhole signalling scheme, because the necessary power is unavailable. Consequently, the repeaters Shawhan discloses operate at intermediate frequencies.
Torsional waves are also discussed as suitable candidates for information transmission. In U.S. Pat. No. 3,588,804, Fort discloses a downhole, ultrasonic transducer for production of waves. Two United States patents to Lamel et al. discuss use of torsional acoustic waves of zero order as the preferred means of signalling to and from a subsurface equipment package. One, U.S. Pat. No. 3,900,827, discloses a crossed-field magnetostrictive transducer suitable for torsional wave generation. The other, U.S. Pat. No. 4,001,773, discloses an improved acoustic communication method utilizing modulated, torsional acoustic waves inherently produced as noise in the drill string by virtue of the drilling operations.
To signal from a surface location to a disaster valve positioned at the bottom end of tubing, Parker discloses three (3) transmitters in U.S. Pat. No. 4,038,632. The tubing is suspended from either a magnetostrictive or a hydraulic ram in two of the transmitter embodiments. Since the lower end of the tubing extends down into the earth, and the upper end of the tubing hangs upon rods in an unrestrained manner, the intermediate tubing is free to move or to stretch. The third transmitter is a tuned acoustic hammer which pounds periodically on the tubing. All of these systems are large, permanent additions to well completions. According to Parker, the pulses of sonic energy resolve into compression and transverse wavefronts which propagate through the pipe at differing velocities. Sensitive to this characteristic time delay in passage of the compression and transverse waves, receivers maintain the disaster valve open until reception of the signal ceases.
Many methods have been investigated. None is a commercially feasible or commercially successful apparatus for acoustic telemetry in rotary drilling operations. Downhole transmitters generate intermediate frequency acoustic waves which are attenuated during transmission in the drill string to a greater extent than low frequency waves. Larger downhole transmitters disclosed to date require substantial modification of the drilling equipment and have not proven to be successful commercially. Transmitting modulated torsional waves has not materialized as a viable alternative, nor has suspending the pipe on rams or permanently affixing a hammer to the pipe. Thus, the search continues for a commercially valuable, low frequency, longitudinal acoustic wave transmitter useful for telemetry operations in rotary drilling.